High-speed Internet access services, such as fiber to the home (FTTH) service, have been in widespread use recently. The FTTH services include passive optical network (PON) in which a single optical fiber is shared by multiple users.
Japanese Laid-open Patent Publication No. 9-18412 discloses a technique that determines a degraded state of a laser diode by comparing a degradation determination voltage with a voltage corresponding to a pre-bias current value occurring immediately before the laser diode (LD) shifts to a laser oscillation state thereof. Japanese Laid-open Patent Publication No. 2005-197984 discloses a technique that determines an LD degradation based on driving current data following temperature characteristics that keep an optical signal output at a specific level, and corrects driving current control data in order to control an optical signal output in an initial operation state to a specific level.